This invention is directed to supersonic flow channels and, more particularly, to electrodes suitable for use in supersonic flow channels. As will be better understood from the following description, this invention is primarily useful as an electrode for a high power, ionizing beam supported, uniform electrical discharge through a supersonic flow.
Supersonic electrical discharge lasers include a supersonic flow channel through which a suitable gas travels at supersonic speeds. Electrodes, formed in the walls of the channel, create high power, ionizing beam supported, uniform electrical discharges through the flow at predetermined intervals. In order for such a laser to operate satisfactorily, the electrode configuration must satisfy four requirements. First, it must not unduly disturb the supersonic flow; second, it must provide a "window" for the ionizing beam; third, it must have a cross-sectional area adequate to conduct large electric currents; and, fourth, it must be strong enough to withstand up to an atmosphere of wall pressure without distortion.
With respect to the first requirement noted above, it has been found that local flow deflection angles cannot exceed about 0.1.degree. because of the magnitude of the density variation through associated waves. Greater angles with stronger waves result in too large an optical path distortion. While a solid flat plate electrode can be readily designed to meet this restriction, a solid flat plate electrode will not satisfy the other denoted requirements, particularily the second requirement. In the absence of a solid wall, pressure becomes the boundary condition that controls flow direction.
Therefore, it is an object of this invention to provide an electrode for a supersonic flow channel that provides a pressure boundary that does not unduly disturb a supersonic gas flow through the channel.
It is a more general object of this invention to provide a solid non-flat plate supersonic flow channel wall that does not unduly disturb a supersonic gas flow through the channel.
In the past, the second and third requirements noted above have been solved by forming the electrode of a uniform array of metal bars or wires located parallel to the flow direction. The bars were thick enough to safely conduct the current and withstand the pressures involved. further, the bars were separated by open spaces. The spaces provide a window for an electron, ultra-violet ray, or X-ray beam, and normally make up at least 70% of the area covered by the entire electrode. In theory, if the static pressure between the bars immediately adjacent to the supersonic stream is matched to the static pressure of the flow to within about 0.75%, the associated flow disturbance will be adequately small. In practice, however, it is difficult to maintain this level of pressure matching because of the high velocity shear layer that exists where the two static pressures meet. More specifically, there is a dynamic interaction through the high velocity shear layer that causes it to entrain the adjacent gas between the bars and pump it out with the flow. The only known way to overcome this problem is to continually resupply gas to the grooves between the bars so as to maintain the desired pressure in these regions. However, resupplying gas to this region presents other problems. Specifically, the degree of pumping of the gas from the grooves varies in the streamwise direction so that different local resupply rates are required for a local pressure match. Thus, there are two main, but related, problems associated with prior art electrodes comprising a series of spaced bars arrayed parallel to the supersonic flow direction. First, the gas between the bars is pumped out with the flow whereby the gas must be continuously resupplied in order to maintain the necessary pressure match. Second, the rate of pumping varies in the direction of flow whereby the gas must be resupplied at different rates along the grooves between the bars.
Attempts to achieve the desired pressure matching have been made; however, they have not been as successful as desired. The forced injection of gas into the grooves has been found to be entirely unsuitable because only about 20% pressure equalization can be achieved. While demand flow through a large opening into a large constant pressure reservoir can provide adequate pressure matching, this technique is also undesirable. Specifically, pressure matching, within a few percent can only be provided for a small fraction of a second using this technique. Moreover, in order to obtain any suitable pressure match at all, the pressure behind the bars (in the pressure reservoir) must be initially higher than the flow static pressure. Then, the discharge must be timed to occur exactly when the shear layer pumping action has created the closest possible pressure match. Moreover, the small streamwise pressure gradient created by the pumping action must be accepted as a limiting factor. Finally, while this technique for pressure matching may be useful when the discharge pulse period is short, obviously, it cannot be used when the discharge pulse period is long. Morever, the recovery period makes this technique unsuitable when rapid repetitive pulsing is desired. In addition, this technique does not result in high flow uniformity.
Another attempt directed to overcoming the foregoing problems was to place a thin foil over the bars, adjacent to the flow, to seal the surface and appear to the flow as a smooth solid wall. It has also been unsuccessful. Nonmetallic foils, sufficiently thin to act as a window, rupture under the starting pressure wave generated by the supersonic flow. In addition, because nonmetallic foils are generally electrically nonconducting, high power discharging is not possible. Also, thin metallic foils provide additional electrode surface for the high power discharge in the areas of greatest ionizing beam intensity. Adequately thin metal foils rupture during such a discharge. Moreover, because of the wide electrode bar spacing required to obtain the desired large window area, thin metal foils dimple and create surface angles much greater than the 0.1.degree. limiting value discussed above. Thus, the intended ideal flow boundary is not realized utilizing a thin foil to form part of the electrode structure where the electrode bars run parallel to the direction of flow and the foil, whether it be metallic or nonmetallic, lies atop the bars.
It has also been found that placing a thin foil behind electrode bars aligned parallel to the direction of flow does not eliminate the production of distorting waves. In this case, shear layer pumping deflects the flow into the shallow regions (grooves) formed between the bars causing it to attach to the foil. The deflected flow results in the production of moderately strong waves in the main flow. A gradual taper upstream of the bars to control the deflection angle and, therefore, the strength of the associated waves is impracticable because the taper would have to start many feet upstream of the electrode in order not to exceed the 0.1.degree. deflection angle limitation discussed above. It is also impracticable to extend the electrode bars upstream into the nozzle.
It can be seen from the foregoing discussion of different types of electrodes generally formed of a plurality of parallel bars aligned with the direction of flow, that such electrodes create a number of problems which made their use generally undesirable.
Therefore, it is also an object of this invention to provide a new and improved electrode suitable for use in an electrical discharge laser.
It is a further object of this invention to provide a low-disturbance grid wall suitable for use as an electrode in a supersonic electrical discharge laser that does not require the inclusion of complicated mechanisms to create pressure matching at the shear layer.
It is yet another object of this invention to provide a low-disturbance electrode suitable for use in a supersonic electrical discharge laser that allows high power and/or rapid electrical discharges to occur in the gas passing through the laser's channel.